chore: remove Sentry integration

Remove @sentry/nextjs and all related files since it was never actively used.
- Delete sentry.server.config.ts, sentry.edge.config.ts
- Delete sentry-example-page and sentry-example-api routes
- Clean up instrumentation.ts, global-error.tsx, middleware.ts
- Remove Sentry env vars from env.example and docs
- Update CLAUDE.md, copilot-instructions.md, PRODUCTION_READINESS.md

Middleware bundle reduced from 86KB to 34.8KB (-51KB).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

app/api/sentry-example-api/route.ts

@@ -1,11 +0,0 @@
-import * as Sentry from "@sentry/nextjs";
-import { NextResponse } from "next/server";
-
-export const dynamic = "force-dynamic";
-
-// A faulty API route to test Sentry's error monitoring
-export function GET() {
-  const testError = new Error("Sentry Example API Route Error");
-  Sentry.captureException(testError);
-  return NextResponse.json({ error: "This is a test error from the API route" }, { status: 500 });
-}